To control a process in an industrial plant using a controller such as a proportional-integral-derivative controller (PID controller), manipulated variables are sent from the controller to a process element under control, such as a valve. Each manipulated variable is intended to adjust a function of the process element in an attempt to reach a desired output. The desired output is typically provided to the controller as a set-point. Process variables reflect the output of the process and are obtained by a sensor associated with the process element. A process variable is provided to the controller as a form of feedback in response to a corresponding manipulated variable, for comparison with the set-point, so that the controller can in turn provide a subsequent manipulated value to the process for adjusting the subsequent process variable to reach the set-point.
It will be appreciated that proper functioning of the controller is essential for correct control of the process, and to that end, model identification of the process is critical for tuning and advance designing of the controller. Model identification for process control is based on process step response data with sufficient excitation, such as the manipulated variables sent from the controller in response to the process variables received. However, the manipulated variables are inevitably not in an initial steady state condition because the plant condition keeps changing or disturbances from other aspects are experienced. Consequently, a manipulated variable with sufficient excitation but not in an initial steady state condition cannot be relied on as a starting point for identifying a good process model. In many cases, the process step data has to be abandoned because of failure to get a good fit of the resulting model with the actual process output obtained. Obtaining process step data for identifying a good process model is also costly because conducting step test on plant is time consuming and can cause plant upset, with risks on loss of product specification.